Thum-See, Bathing lake in Karlstein, Bad Reichenhall, Germany

Thum-See is a lake about one kilometre long and 325 metres wide in the Berchtesgadener Land district, lying some three kilometres west of Bad Reichenhall's centre and in private ownership. Its deepest point is 21 metres below the surface. On the eastern shore, beside the 4,599 m² marshy pond called Seemösl or Seerosensee, stands a water-lily farm laid out in 1936.

The area gained recognition as a mountain retreat in the 19th century, becoming more widely visited in the following decades. Development of facilities around the lake grew to serve growing interest in alpine recreation and summer activities.

The name Thumsee was first recorded in 1563 as Dumbsee; it may derive from Middle High German tumb (mute) or Latin dumosus (overgrown with scrub). East of the lake, the Seemösl pond holds Germany's oldest water-lily farm, laid out in 1936, where nine varieties were once cultivated; it is no longer tended, so hardly any lilies bloom today.

The lake is reached by a city bus line of the Bad Reichenhall municipal works, and a circular hiking trail runs around it. The public lawn and the Thumseebad with its terrace café serve bathers in summer, while two excursion restaurants stay open year-round for walkers.

East of the lake, beside the 4,599 m² marshy pond called Seemösl or Seerosensee, lies a water-lily farm laid out in 1936, the oldest in Germany. Nine varieties were once cultivated there; since the site is no longer tended, almost no lilies bloom today.